Hello Nathan,
On Thu 23-01-20 09:56:10, Nathan Lynch wrote:
> Libor Pechacek <lpecha...@suse.cz> writes:
> > In KVM guests drmem structure is only zero initialized. Trying to
> > manipulate DLPAR parameters results in a crash in this environment.
>
> I think this statement needs qualification. Unless I'm mistaken, this
> happens only when you boot a guest without any hotpluggable memory
> configured, and then try to add or remove memory.
Thanks for the review. The introductory statement can indeed be clearer.
[...]
> > diff --git a/arch/powerpc/platforms/pseries/hotplug-memory.c
> > b/arch/powerpc/platforms/pseries/hotplug-memory.c
> > index c126b94d1943..4ea6af002e27 100644
> > --- a/arch/powerpc/platforms/pseries/hotplug-memory.c
> > +++ b/arch/powerpc/platforms/pseries/hotplug-memory.c
> > @@ -236,9 +236,9 @@ static int get_lmb_range(u32 drc_index, int n_lmbs,
> > if (!start)
> > return -EINVAL;
> >
> > - end = &start[n_lmbs - 1];
> > + end = &start[n_lmbs];
> >
> > - last_lmb = &drmem_info->lmbs[drmem_info->n_lmbs - 1];
> > + last_lmb = &drmem_info->lmbs[drmem_info->n_lmbs];
> > if (end > last_lmb)
> > return -EINVAL;
>
> Is this not undefined behavior? I'd rather do this in a way that does
> not involve forming out-of-bounds pointers.
Well, this is a tough question for the case when drmem_info->lmbs was not
allocated. Given that the array does not exist, what bounds are we talking
about?
My patch builds on the fact that NULL[0] is NULL and NULL < NULL is false.
Talking about a pointer to one past the last element of an non-existent array
is too much philosophy for me.
For the case when drmem_info->lmbs is allocated, last_lmb is a pointer to one
past the last element of the array as Michal mentioned.
> Even if it's safe, naming that pointer "last_lmb" now actively hinders
> understanding of the code; it should be named "limit" or something.
Good catch.
[...]
> 1 file changed, 36 insertions(+), 7 deletions(-)
> arch/powerpc/include/asm/drmem.h | 43 +++++++++++++++++++++++++++++++++-------
>
> modified arch/powerpc/include/asm/drmem.h
> @@ -20,19 +20,48 @@ struct drmem_lmb {
>
> struct drmem_lmb_info {
> struct drmem_lmb *lmbs;
> - int n_lmbs;
> + unsigned int n_lmbs;
> u32 lmb_size;
> };
>
> extern struct drmem_lmb_info *drmem_info;
>
> -#define for_each_drmem_lmb_in_range(lmb, start, end) \
> - for ((lmb) = (start); (lmb) <= (end); (lmb)++)
> +static inline bool drmem_present(void)
> +{
> + return drmem_info->lmbs != NULL;
> +}
Yes, use of this test was also my first idea about the fix.
> +static inline struct drmem_lmb *drmem_lmb_index(unsigned int index)
> +{
> + if (!drmem_present())
> + return NULL;
>
> -#define for_each_drmem_lmb(lmb) \
> - for_each_drmem_lmb_in_range((lmb), \
> - &drmem_info->lmbs[0], \
> - &drmem_info->lmbs[drmem_info->n_lmbs - 1])
> + if (WARN_ON(index >= drmem_info->n_lmbs))
> + return NULL;
Why is this WARN_ON needed?
> +
> + return &drmem_info->lmbs[index];
> +}
> +
> +static inline struct drmem_lmb *drmem_first_lmb(void)
> +{
> + return drmem_lmb_index(0);
> +}
> +
> +static inline struct drmem_lmb *drmem_last_lmb(void)
> +{
> + if (!drmem_present())
> + return NULL;
> +
> + return drmem_lmb_index(drmem_info->n_lmbs - 1);
Is the unsigned integer wraparound intended in drmem_info->n_lmbs == 0 case?
> +}
> +
> +#define for_each_drmem_lmb(lmb)
> \
> + for ((lmb) = drmem_first_lmb(); \
drmem_first_lmb() is essentially a call to drmem_info->lmbs(0). What
happens if drmem_info->n_lmbs is zero and drmem_info->lmbs is not NULL?
> + (lmb) != NULL && (lmb) <= drmem_last_lmb(); \
> + (lmb)++)
> +
> +#define for_each_drmem_lmb_in_range(lmb, start, end) \
> + for ((lmb) = (start); (lmb) <= (end); (lmb)++)
>
> /*
> * The of_drconf_cell_v1 struct defines the layout of the LMB data
>
After all, I don't mind how the bug will be fixed. As you can see, my
preference is towards simpler solutions.
In my opinion your solution special-cased drmem_info->lmbs == NULL and opened
the doorway to the combination of drmem_info->lmbs != NULL &&
!drmem_info->n_lmbs. Maybe the condition can never become true but the code
should IMHO be robust enough to handle it.
Thanks!
Libor
--
Libor Pechacek
SUSE Labs Remember to have fun...
